As I understand it, those that have migrated to FiRST only use low band when they're out of range or having other issues with the system. Otherwise, keep in mind that MSP uses their 39 Mhz system in simplex only - meaning they're not using repeaters. This makes them more difficult to hear, and they aren't very chatty as some PDs are.
I understand that a few are patched from 39 Mhz to FiRST but not everywhere.
Mike